CNC Machining material choice: matching alloys to strength and machinability needs
Content Menu
● Strength Properties That Matter in Machined Parts
● Machinability – What It Really Costs You
● Everyday Alloys and Where They Actually Get Used
● Practical Strategies That Save Time and Money
● Four Real Jobs and the Material Decisions Behind Them
Introduction
Material selection in CNC work is rarely simple. Strength requirements come from the design engineer, machinability concerns come from the shop floor, and cost pressure comes from management. All three sit in the same meeting and expect the manufacturing engineer to find the one alloy that satisfies everyone. Most days there is no perfect answer, only the least painful compromise.
Over the past fifteen years of programming and running CNC mills and lathes, the same alloys keep showing up on drawings: a handful of aluminums, a longer list of steels, titanium when the part has to survive extreme conditions, and occasionally something exotic that makes everyone nervous. The goal here is to walk through the real trade-offs that actually matter when the spindle starts turning, not just the numbers on a datasheet.
Strength Properties That Matter in Machined Parts
Tensile strength, yield strength, and fatigue strength drive most structural decisions. A part that sees static load cares mostly about yield strength. Anything cyclic—shafts, gears, aircraft fittings—lives or dies by fatigue limit.
7075-T6 aluminum reaches 503 MPa yield and still weighs only 2.8 g/cm³. That combination explains why landing-gear beams and rocket motor cases are almost always 7075. The same part in 6061-T6 would be thicker, heavier, and still weaker at 276 MPa yield.
4140 quenched and tempered to 32–36 HRC delivers roughly 930 MPa tensile strength with decent toughness. Transmission input shafts, crankshafts, and bolster plates on presses run 4140 because the part has to take shock loads without snapping.
Ti-6Al-4V Grade 5 sits at 880–900 MPa tensile strength, keeps most of that strength to 400 °C, and resists corrosion in body fluids or seawater. Hip stems, compressor blades, and subsea hydraulic couplers are built from it even though every machinist groans when the material shows up on the traveler.
Machinability – What It Really Costs You
Machinability is measured in minutes of tool life, dollars per part, and how many times the operator has to change an insert on a Friday afternoon.
6061-T6 machines at 300–400 m/min with sharp carbide and leaves a surface most polish shops will accept straight off the machine. A 12 mm three-flute end mill running 0.10 mm/tooth at 6000 rpm is routine.
7075-T6 drops surface speed 25–30 % and needs more frequent peck cycles because zinc makes it work-harden. The same 12 mm cutter usually runs 4500 rpm and 0.07 mm/tooth or you watch the built-up edge grow.
4140 pre-hard (285–320 HB) takes 120–150 m/min with coated carbide. Push harder and the insert corners round off fast. Most shops run conservative speeds and still get acceptable tool life because the material is predictable.
Ti-6Al-4V is a different world. Typical parameters are 45–60 m/min, 0.05–0.08 mm/tooth, and high-pressure coolant through the tool. A 10 mm end mill might last 40–60 minutes before the flank wear hits 0.3 mm. Double that tool cost is normal compared to aluminum.
Everyday Alloys and Where They Actually Get Used
Aluminum
6061-T6 – enclosures, heat sinks, fixture plates, robot base plates. Easy, cheap, and strong enough for 90 % of general engineering.
2024-T3 – aircraft wing skins and spars where fatigue matters more than corrosion resistance.
7075-T651 – mountain-bike suspension links, firearm receivers, satellite brackets. High strength, but plan on stress relieving after roughing.
5083-H116 – marine camera housings and weldments that need saltwater resistance without paint.
Carbon and Alloy Steels
1018 / 1045 – prototype brackets, soft jaws, low-stress shafts. Machines fast, welds easy, cheap.
4140 HT – hydraulic cylinder rods, gear blanks, forging dies. Good combination of strength and toughness.
4340 – landing-gear retract actuators, high-load spline shafts. Needs vacuum arc remelt for aircraft work.
D2 – punch and shear blades. Hard to machine (30–40 m/min), but holds an edge forever.
Stainless Steels
303 – Swiss-turn screws and fittings. Sulfur makes it cut like brass.
304/316 – food-processing equipment, medical trays. Sticky chips, work-hardens, but corrosion resistance is mandatory.
17-4PH Condition H900 – valve bodies, pump shafts, aerospace actuators. 1100 MPa strength and still machines reasonably at 150–180 m/min.
Titanium and High-Temp Alloys
Ti-6Al-4V ELI – knee and hip implants, deep-diving submersible parts.
Grade 2 CP titanium – chemical plant heat-exchanger baffles where strength is secondary to corrosion.
Inconel 718 – turbine disks, rocket thrust chambers. Machines at 20–30 m/min with ceramic or whisker-reinforced tools.
Copper Alloys
C101 OFHC – electrical bus bars, spot-welding electrodes.
C36000 brass – plumbing fittings, pneumatic connectors. Still the fastest-cutting alloy most shops ever see.
CW510L (lead-free brass) – drinking-water valves in Europe and California. Slightly slower than C360 but meets regulations.
Practical Strategies That Save Time and Money
Rough in the annealed or solution-treated condition whenever possible. 7075 solution-treated is 150 HB instead of 150 HB after aging. Ti-6Al-4V annealed runs 20–25 % faster than aged material.
Leave 0.5–0.8 mm for finish passes after heat treat. Distortion is real, especially on thin walls.
Use high-pressure coolant on titanium and stainless. 70–100 bar through the tool can double insert life.
For 17-4PH and Inconel, climb mill whenever geometry allows; conventional milling causes more work hardening.
Run vibration-damping boring bars on long cantilevered tools. A 6×D bar in 4140 will chatter at 0.15 mm/rev unless damped.
Four Real Jobs and the Material Decisions Behind Them
Drone Motor Mount
Required 450 MPa minimum yield, total part weight under 180 g. 7075-T6 chosen. Machined from 50 mm plate, rough at 500 m/min, finish at 380 m/min, stress relieved between rough and finish. Tool cost high, but part passed drop tests from 20 m.
Hydraulic Cylinder Rod
68 mm diameter, 1.2 m long, 100 kN buckling load. 4140 pre-hard chosen. Turned between centers, then milled keyways. Ran 180 m/min with CNMG inserts, 0.35 mm/rev. Total cycle 18 minutes per rod.
Bone Plate for Trauma Surgery
Ti-6Al-4V ELI required for biocompatibility and fatigue. 3 mm thick, 180 mm long, twelve 4.5 mm holes. Milled with 8 mm variable-helix end mill at 55 m/min, 0.06 mm/tooth, 80 bar coolant. One insert per three plates.
Water Meter Body
Originally C844 leaded bronze, had to switch to CW511L lead-free. Same 350 MPa tensile, but chip breaking worse. Added 8 % higher spindle load, changed to positive-rake ISO inserts with chip curlers. Cycle time up 14 %, but now ships to EU without restriction.
Conclusion
The right alloy is the one that lets the part survive its real-world loads while still being producible on the machines and budget you actually have. 6061 will handle most prototype and light-duty jobs without drama. Step up to 7075 or 4140 when strength or fatigue becomes the limiting factor. Accept that titanium and Inconel cost more per minute but buy performance nothing else can match.
Every shop eventually builds its own short list of “go-to” materials that the programmers and operators already understand. The fastest way to expand that list is to run controlled tests—same tool, same holder, different alloys—and log tool wear, surface finish, and cycle time. Data beats opinion every time.
Next time a new drawing lands on your desk, start with the required mechanical properties and environment, then work backward to the most machinable grade that still meets the spec. The machine will thank you, the accountant will thank you, and the part will still be in one piece when the customer needs it.
Frequently Asked Questions
Q1: Will 2024 aluminum replace 7075 in new aerospace designs?
A: Rarely. 2024 has better fatigue crack growth resistance but lower static strength and worse corrosion behavior unless clad. Most new structural designs still specify 7075-T73 or 7050-T7451.
Q2: Is it worth paying extra for pre-hardened 4140 versus annealing 4140 yourself?
A: Yes on small-to-medium lots. The mill supplies uniform 28–32 HRC, you skip the annealing furnace, and distortion after machining is lower.
Q3: How much slower is lead-free brass than C360 in practice?
A: 12–18 % longer cycle time on average if you keep the same surface finish target. The gap narrows with modern coated inserts and higher pressure coolant.
Q4: Can I machine Ti-6Al-4V dry if the shop has no coolant system?
A: Possible on very light cuts (0.02 mm/tooth, 40 m/min) with sharp PCD-tipped tools, but tool life drops 70–80 %. Most shops add at least mist or chilled air.
Q5: When should I consider 7050 aluminum instead of 7075?
A: Thick sections over 100 mm where quench sensitivity matters. 7050 maintains strength better through the core and has lower residual stress.